Sintered silicon nitride articles, due to their inherent resistance to heat, heat shock, and wear, have found applications as constituent materials in a variety of high temperature equipment and as cutting tools. However, the high heat resistance of silicon nitride also makes it difficult to sinter. It is generally fired or sintered using sintering aids. Such sintering aids have a softening point that is lower than the decomposition temperature of the silicon nitride, and, during firing, they exist in the glass phase at the grain boundaries. When such sintered articles are used at high temperatures, the grain boundaries soften to a glass phase, entailing a decline in their properties such as wear resistance.
In order to resolve the foregoing problems, proposals have been made to use sintering aids or adjuncts that have a high softening point (see Japan Patent Application "Kokai" Disclosures Hei 4-209763, Hei 4-240162, and U.S. Pat. No. 5,382,273). Other disclosures also suggest that reducing the total amount of the sintering aids is effective in reducing the glass phase grain boundaries. However, in general, when high melting-point sintering aids are employed, the sintering properties are inferior to those obtained by using low melting-point sintering aids. As a result, in order to obtain high density sintered articles using high melting-point sintering aids, large amounts of such sintering aids must be incorporated, even at some sacrifice to the high temperature properties of the resulting articles.
Oxides of rare earth elements are well known as sintering adjuncts that form glass-phase grain boundaries with high softening points when sintering silicon nitride materials, Since the chemical properties of the rare earth elements are similar to each other, in most cases they are accorded similar treatment as a group. However, when considering them as sintering aids for silicon nitride, their sintering properties are different from each other, and the properties of the resulting sintered articles are not necessarily the same.